Why individual resistors must be used, rather than having just one resistor in series with the common terminal of the display.
The display here is represented abstractly using just five LEDs.
The goal is to operate the simulator, and try all the combinations of the switches, and watch the total current in the system (near the ground terminal on the lower right).
It is meant to convince someone skeptical of this requirement (cannot and must not omit the individual resistors).
Simulator:
https://falstad.com/circuit/avr8js/
same image as above:
END.